[NOTE: Whenever I write about actual cosmic events that might possibly affect us on Earth, I get scared emails from some folks. So let me be up front: there are no stars close enough to Earth to hurt us should they explode. Nothing I write in this post changes that; I’m talking about a star that can go supernova that’s closer than I thought any was, but still much too far away to do much to us. So don’t panic. But do please enjoy the over-the-topness of what happens when a star explodes. Because it’s cool.]

On May 13 I tweeted this one: BAFact: A supernova has to be less than about 75 light years away to hurt us. No star that close can explode, so we’re OK. The distance may actually be somewhere between 50 – 100 light years, and it depends on the kind of exploding star, but I have to keep these factoids to about 110 characters to tweet them. Nuance is at a premium.

I got so many replies about that one that I decided to do a theme week, and stick with supernovae. The next day I tweeted this: BAFact: The nearest star that can go supernova is Spica – it’s 260 light years away, so we’re safe, and I linked to a video I did a few years back this.

A few minutes later I got a tweet from Nyrath, saying that he thought the nearest star that could explode was IK Pegasi, 150 light years away.

I looked this up, and here’s the thing: he’s right! I had never heard of IK Peg, so I didn’t even know it existed. And it turns out it is the nearest star that can explode, though technically it probably isn’t.

And you know when I say something weirdly oxymoronic like that there must be a good story here, right? Mwuhahahaha. Yes. yes, there is. Stick with me; this is long, but also awesome.

The story

It’s been known for a while that IK Peg is a weird star (you can read quite a bit about it on the ESO website, though the formatting is a bit messed up). It looks like an A-type star — that is, more massive, hotter, and bigger than the Sun. It’s not nearly enough to explode — stars need to be at least 8 times the Sun’s mass to do that, and this star is only about 1.7 times heftier than the Sun.

It pulsates, getting brighter and dimmer on a pretty rapid timescale: each cycle only takes about an hour. A lot of stars do this, but typically when one does it means it’s nearing the end of its life. In a few dozen million years it’ll swell up into a red giant, blow out a strong wind that’ll strip its outer layers away (creating a gorgeous planetary nebula), and eventually retire as a white dwarf; small, dense, and hot, cooling slowly over billions of years.

Except… there’s a monkey in the wrench. The star isn’t alone.

It has a companion. And this is where things get interesting.

IK Peg B

Years ago, the Germans launched the ROSAT satellite designed to survey the entire sky looking at objects that emit extreme ultraviolet light (this is the one that burned up in late 2011 over the ocean). UV emitters tend to be hotter objects, like massive stars and gas. At the position of IK Peg they detected a strong UV source. A-type stars aren’t nearly this bright in the UV, so it was clear something was up.

In fact, it had been known for a long time that IK Peg had an invisible companion — there was a periodic Doppler shift in the spectrum, which means it was sometimes approaching us and sometimes receding, as you’d expect for something in orbit around something else.

So this companion is bright in UV, faint in visible, and massive enough to cause that Doppler shift in the A star. Its mass is too low to be a black hole or a dense neutron star, so it must be a white dwarf. These are very hot, so they blast out ultraviolet, but so small that they’re faint in visible light. Interestingly — very interestingly, as we’ll see in a moment — this companion star is pretty high-mass for a white dwarf, about 1.1 to 1.2 times the mass of the Sun. Usually, white dwarfs are around 0.6 times the Sun’s mass, and it’s rare to find one that heavy.

For future reference, in binary systems, the brighter of the two stars is given the designation A, and the dimmer B. So the A-type star is IK Peg A, and the white dwarf is IK Peg B.

The history of a lover’s embrace

White dwarfs, as I mentioned above, are the leftover cores of stars like the Sun that lose their outer layers. The Sun will become one in about 6 -7 billion years, when it uses up its supply of hydrogen fuel in its core. This means the IK Peg system is old. And just knowing what these two stars are now, we can unravel what they were like long ago.

A long time ago, billion of years ago, there were two stars orbiting each other. One was much like the Sun, but the other more massive, probably 3 – 4 times the Sun’s mass. More massive stars live shorter lives, so this star blew through its fuel, and expanded into a red giant (the details of this are complex, but you can get a synopsis here).

When I say expanded, I mean it: it got huge, so big it enveloped the other star. This is called a common envelope system — and think about that: we had a whole star physically inside another star! During this period the lower-mass star actually gained mass, drawing material from its bigger companion.

As this was happening the red giant was blowing out a strong wind of matter, which, over time, leeched away the outer part of the star. What was left was a hot white dwarf — IK Peg B — and a normal companion star — IK Peg A — that was now somewhat more massive than it was before. Impressively, we can detect all kinds of elements in the atmosphere of IK Peg A that shouldn’t be there; these are what it sucked away from IK Peg B while it was still a red giant.

And there you have it. That’s the system we see today.

When a relationship blows up

OK, cool. But of course, not even the most stable relationship lasts forever.

IK Peg A is aging. It’s still fusing hydrogen into helium in its core like the Sun does. But remember, those pulsations are telling us it’s nearing the end of its life too. At some point in the future, probably in a few dozen or hundred million years, it too will swell into a red giant.

When it does, the reverse of what happened before will occur. Material from IK Peg A will flow onto the white dwarf. Separated by a mere 30 million kilometers or so (closer than Mercury is to the Sun), this transfer of mass will flow steadily. As the matter piles up on the surface of the white dwarf it gets fiercely compressed and hot. At some point the temperature gets high enough to flash fuse it into helium. There will be an explosion — big, but not big enough to destroy the star — called a nova. Some of the hydrogen will remain, as will the helium. When things calm down, the material from the red giant will start to pile up again.

Lather, rinse, repeat.

But every time it does this, not all the added material blows away. The mass of the dwarf increases. It’s also possible that the matter from the red giant will accumulate slowly enough that it will pile up without a nova explosion. Either way, the mass of the white dwarf increases. And remember, IK Peg B is already pretty massive. It can only gain so much more mass before something very bad happens…

One day, something very bad happens. When the dwarf reaches a mass of about 1.4 times the Sun, the physical forces inside the star can no longer support its own mass. The white dwarf starts to collapse, and the core temperature rockets skyward. A fusion chain reaction is ignited in the dwarf, and the conditions inside it cannot stop it. Within seconds, the chain reaction runs out of control, consuming the bulk of the star, and it explodes.

And oh, that explosion. Blasting out of the living hell that was once a normal star is so much energy it can outshine an entire galaxy. In seconds the newly-born supernova releases as much energy as our Sun will over its entire lifetime.

It’s a supernova.

And when it finally fades, weeks later, there may be a dense neutron star left over, or it’s possible the star will have blown itself to bits. But either way, the mind-numbing energy is on its way, spreading out into the galaxy.

A whelk’s chance

Supernovae are among the most violent events in the modern Universe. You don’t want to be near one!

Which is something of an issue. IK Peg is only 150 light years away.

Now, don’t panic! That’s probably far enough away that the damage to Earth would be minimal. The flood of ultraviolet and higher-energy light might affect our ozone layer, but from my reading on this — which is extensive, since I wrote a chapter in my book Death from the Skies! about this and went through dozens of journal articles — 150 light years is far enough to dim the effects substantially.

And we’re safer than that. Measurements of the system’s velocity shows it to be moving away from the Sun at about 20 km/sec. That’s about a light year per 16,000 years… and remember, IK Peg A won’t go red giant for probably millions of years! They’ll be 60 light years farther away for every million years in the future the system sticks around, and if IK Peg A can hold its breath a little longer, it could easily be a thousand light years away before it goes. At that distance a supernova is no danger for sure.

The closest supernova?

Which leaves me in a funny spot. Right now, IK Peg is the closest potential supernova. But by the time it goes off, it won’t be! Spica is close right now, as I pointed out before, but in a few million years all these stars will move. What’s close now may not be by the time one of them goes supernova, so the idea of which potential supernova is closest doesn’t have a lot of meaning. The better question is "which one that goes off next is the closest?" and there’s no real way to answer that. Predicting just when a supernova will explode isn’t possible.

But I think that’s OK. What we’re really asking is, what stars near us can explode? And it turns out the answer is that there are a lot within a thousand light years, a handful within 500, and none within the hard 50 – 100 light year limit.

In other words: you can take supernovae off your things-to-be-scared-of list. They’re all too far away, at least for the next few million years.You can breathe easy.

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Since we’re talking about white dwarfs, what about Sirius B? It’s at 0.98 solar masses. Granted Sirius A (also A type) has probably has approx 1 billion years left, so the Earth will be uninhabitable before that, but couldn’t it be a closer supernova? Or is it’s orbit too far out (~50 years) that the amount of matter it accretes is minimal.

“Years ago, the Germans launched the ROSAT satellite designed to survey the entire sky looking at objects that emit ultraviolet light”

ROSAT is ROentgen SATellit, Röntgen being the German name for X-rays (named after the discoverer, of course). It’s primarily known as an X-ray satellite. Perhaps that is “extreme ultraviolet” by some definition.

Wait, if 1Kpeg A and B originally orbitted each other within the orbit of Mercury, wouldn’t that have been like the stars practically touching when they were both fresh-faced youngsters? That must have been quite the sight back in the day.

Although Alpha Canis Minoris as its also known is one spectral class later in nature – F rather than A – but similar case again.

I think both Sirius and Procyon are too far from their respective companions to go supernovae but, not 100% sure of that and both Dogstar “pups” are certainly a long way from exchanging mass or exceeding their Chandrasekhar limits which won’t happen for hundreds of milions of years even if Procyon A is beginning its sub-giant phase as sometimes listed.

I vaguely recall another SF novel (or two with a sequel?) featuring Alpha Centauri going supernova but that prospect can be very safely ruled out – a fact I think the author got around by having aliens blow it up!

Remember that the thermonuclear supernovae of a white dwarf is much brighter than core-collapse supernovae from a massive star, although with a much smaller neutrino flux, so the kill zone is different. I think there is a lot of disagreement about whether the novae blow off more mass than is accreted or less meaning that the white dwarf can never reach the Chandrasekher limit through this method and so do Type 1a supernovae originate from the accretion onto a white dwarf as described above or from two merging white dwarfs?

The orbit within the orbit of Mercury is once B already turned into a white dwarf. Originally they would have been slightly farther apart so that during normal life they would not be close to touching.

During the common envelope phase Phil mentioned, the viscous forces of being engulfed in the other star’s gas (after it swelled up to a giant, so that they can touch now) causes them to spiral closer together into a tighter orbit, which is what we see now that the envelope itself was ejected away.

It always amazes me that people are so worried about “astronomically” improbable ways of dying, but they are probably blissfully unaware of the dangers of dying while driving a car, flying a plane, walking down the stairs, food poisoning, getting shot, not getting vaccinated, radon, CO poisoning, lightning, flood, tornado…. Anyway hope to make them feel a little better with the above information.

They did not touch each others, the distance is too high. Our sun has a diameter of less than 1.5 million kilometers, much less than 1 million km in radius. Even if a 4 solar mass star has a higher radius, it will be far less than 15 million km.

So in fact A and B are near in the sky, and visibly touch (occult) each others. But from the best position you can still look between them.

So you have this couple of stars, bound for life. As one star gets old and starts to die, it gives mass to its partner, which causes the partner to age faster and die sooner, essentially poisoning it. Then, when the partner starts to die, it gives the “poison” back, which can cause the first partner to explode (an explosion which might rip apart the second partner, too)

So even stars can get stuck in dysfunctional and self destructive relationships!

What about the reverse situation – a supernovae candidate that is too far away right now to harm us, but is moving towards us, such that a few million years from now itched it goes off it will be close enough to do harm.

I don’t understand. The sirius system less than 10 light years away. Can you really be all that sure that it’s white dwarf cannot ever possibly explode?

Also, can’t gamma-ray bursts have significant effects from a long way off? I though I read somewhere that there was a GRB that caused ionization in the upper atmosphere. What if a GRB when off inside the Milky Way and we were directly in its beam?

@Christ, #16, I’m more concerned about the really good view we’d have of a supernova than of the miniscule hazard it might present.

@Rene Marie Jones, #22, we’d have to have the star’s pole facing us for a GRB “beam” to strike us. That is already quite improbable, then add in stellar motion of the stars in question moving away from us raising the distance between us and the unlikelihood of the stars occluding each other from our view and one (let alone both) having its pole facing us (boatloads of math and observations are against such a thing being likely), it’s pretty much eliminated as a threat. OTHER stars might be a potential threat, but again, distance is our friend, thanks to the inverse square law.
Phil covered it quite well here and in his original blog AND in Death from the Skies.
Frankly, I’d be more worried about a toilet seat from a space station landing on me than a GRB or novae harming the Earth.

So if the less massive star in a common envelope system gradually spirals inward, would it be possible for one to spiral in far enough to have the cores merge? It seems like the closer it got, the more drag it would have, so it would spiral in faster. What would happen to they system at that point? The red giant would suddenly have an influx of hydrogen into its core, which should trigger a massive increase in its output. Does it explode? Does it just heat up enough that its core expands and is no longer dense enough for helium fusion, so it goes back to the main sequence for a while?

It may also be interesting to consider various other classes of eruptive variables.

One interesting class are the “luminous red novae”, of which the most famous is V838 Monocerotis (look it up, some pretty spectacular pictures of the light echoes from this event have been taken by Hubble). At least some of these are the result of the final merger of contact binary stars: there is actually photometry of the progenitor of the V1309 Scorpii eruption that reveals that it was a ~1.4 day contact binary. The nearest contact binary star is 44 Boötis B, at a distance of 42 light years.

On the other hand, the V838 Monocerotis progenitor was probably around 8 solar masses, so when 44 Boo B merges it will probably be far less energetic.

@ ^ Hemo_jr : The BA has written about Wolf Rayet 104 before assessing its danger – click on my name here for link or see “WR 104: A nearby gamma-ray burst?” posted in 2008 on March 3rd at 11:50 AM.

WR-104 is about 8,000 light years distant although there are uncertainties in this and other estimates have it perhaps as close as 5,000 ly off. Which I guess makes it NOT the closest potential supernova to us!

@9. MTU (me) :

I vaguely recall another SF novel (or two with a sequel?) featuring Alpha Centauri going supernova but that prospect can be very safely ruled out – a fact I think the author got around by having aliens blow it up!

Found that sequel on my bookshelves – ‘Aftermath’ by Charles Sheffield (Bantam Spectra 1998.) with the original being titled ‘Starfire’ apparently, if anyone was curious.

@24. Keith Hearn :

So if the less massive star in a common envelope system gradually spirals inward, would it be possible for one to spiral in far enough to have the cores merge? It seems like the closer it got, the more drag it would have, so it would spiral in faster. What would happen to they system at that point? The red giant would suddenly have an influx of hydrogen into its core, which should trigger a massive increase in its output. Does it explode? Does it just heat up enough that its core expands and is no longer dense enough for helium fusion, so it goes back to the main sequence for a while?

Stellar cores certainly merge in the case of contact binaries – W Ursae Majoris type stars – like 44 Bootis B mentioned by Andy (#25) although these aren’t red giants. The two stars essentially merge into one fast spinning one becioming FK Comae Berenices type stars. I suspect the outer red giant atmosphere would be ejected but not sure.

Since we’re talking about white dwarfs, what about Sirius B? It’s at 0.98 solar masses. Granted Sirius A (also A type) has probably has approx 1 billion years left, so the Earth will be uninhabitable before that, but couldn’t it be a closer supernova? Or is it’s orbit too far out (~50 years) that the amount of matter it accretes is minimal.

Co-incidentally it turns out this question was also asked and answered on the WR-104 thread. Someone called Barton Paul Levenson responded to the idea of Sirius or Procyon B going supernova by pointing out the lack of mass, too distant separation, time factor involved and more. Belated thanks to him again. (I was posting as StevoR at that time many years ago.)

BTW. The Bad Astronomer has also written a great article on another oft cited potential type Ia white dwarf SN threat sometims feared – T Pyxidis – see “No, a nearby supernova won’t wipe us out” posted on the 7th January 2010 at 1:30 PM.

Agreed that Spica appears to be the nearest core-collapse (Type II) supernova progenitor to the Sun (i.e. massive star that will likely end its life as a supernova), but yes, it will be further away than it currently is when it blows up.

I estimate an HR diagram position of: effective temperature ~ 24500 K, luminosity ~ 10^4.25 Suns.
Using the Bertelli et al. (2009) tracks for adopted protosolar abundances of Y = 0.26 (helium mass fraction) and Z = 0.017 (metals mass fraction), this HR diagram translates to a mass of about 12.1 solar masses with age of 14 million years. The lifetime of such a star should be 20 million years, hence it has about 6 million years left. This ignores the effects of Spica’s companion – another massive star of ~7 solar masses that orbits Spica A in ~4 days (not sure if a prediction of the effects of the binary has been worked through in the literature).

Using the revised Hipparcos astrometry from van Leeuwen (2007), and the systemic velocity for the Spica binary from Shobbrook (1972), I estimate the heliocentric velocity in Galactic coordinates to be: U, V, W = -7.7, -16.6, -4.9 km/s.

Using an epicycle orbit approximation code for predicting the past and future orbits of the Sun and Spica (and adopting the Local Standard of Rest velocity from Schonrich et al. 2010, and Oort A and B constants from Feast & Whitelock 1997), I find that Spica should be ~140 parsecs away from the Sun when it blows up in ~6 million years.

So Spica should be no threat.

I went through similar calculations for some other “bombs” currently located within 100 parsecs, and they too should be further away from the Sun than they currently are when they blow up. I use a lower mass threshold value of 8 solar masses (adopted from the recent review by Smartt 2009). The stellar ages, lifetimes, and “t-minus” values assume single star evolution (ignoring the effects of companions stars, which some of the stars have) — so they are very rough calculations. Also, there is still plenty of modern research regarding the effects of stellar rotation, internal mixing, etc. – all of which affects stellar lifetimes. So these numbers are pretty uncertain.

Beta Cru is currently ~85 parsecs away, with a mass of 14 solar masses and age of 10 Myr. Stars of this mass should die on a timescale of 16 million years. Extrapolating the motions of Beta Cru and the Sun in the future shows that they should be separated by about 170 parsecs when Beta Cru eventually blows up.

Eta Cen is currently ~94 parsecs away, with a mass of 9 solar masses and age of 21 or 22 Myr. Stars of this mass should die on a timesclae of ~35 million years. When Eta Cen blows up ~13 million years from now, the Sun should be about 300 parsecs away.

Alpha Mus is currently ~97 parsecs away, with a mass of ~8.6 solar masses and age of ~19 Myr. The star should have a total lifetime of ~38 million years. The Sun should be about ~470 parsecs away from Alpha Mus when it blows up about 19 Myr from now.

All three of these stars (plus Spica, indirectly, and Alpha Cru – for which I haven’t run the numbers yet) are related to the Sco-Cen OB association – the nearest region of “recent” massive star formation. The Sun is moving at about ~20 kilometers per second with respect to the Sco-Cen complex (or about 20 parsecs per million years). The Sun is flying the local Galactic neighborhood, and so some of the nearest “bombs” in Sco-Cen will be further away than they currently are when they eventually explode.

Bellatrix is the other obvious supernova progenitor within 100 parsecs (on the other side of the sky from the Sco-Cen stars). It currently lies 77 parsecs away, with an approximate mass of 9.6 solar masses and age of 20 Myr. The star’s total lifetime should be about 30 Myr. When it blows up in 10 Myr, the Sun will be about 280 parsecs from Bellatrix.

So a rough census of the massive stars within 100 parsecs suggests that none of *these* should be a threat to Earth. Given the Sun’s velocity with respect to young stars in the solar neighborhood, it is possible that the “next” core-collapse supernova to affect Earth may come from a massive star that hasn’t even been born yet, millions of years in the future, somewhere amongst a future star-forming molecular cloud region (that hasn’t even coalesced yet) along the Sun’s projected orbit.

Bomb, this is Lt. Doolittle. You are *not* to detonate in the bomb bay. I repeat, you are NOT to detonate in the bomb bay!

Bomb#20: In the beginning, there was darkness. And the darkness was without form, and void.
Boiler: What the hell is he talking about?
Bomb#20: And in addition to the darkness there was also me. And I moved upon the face of the darkness. And I saw that I was alone. Let there be light.